The present invention relates to a method and a system for laser welding of overlapped metal sheets.
The welding of overlapped metal sheets by means of a laser beam has been already proposed since a long time, particularly in the automotive industry, for the production of motor-vehicle bodies or parts thereof, in lieu of the more conventional electric spot welding. After an initial increasing spread of the laser welding technique approximately starting from 1990 (see for example European patent EP-B-0 440 001 in the name of Comau), the application of this technology in the automotive industry has seen a stop due to the increasing use in a motor-vehicle bodies of zinc plated sheets, i.e. sheets coated with a protective zinc layer for example deposited by galvanization in motor-vehicle bodies. Zinc has a melting temperature of 419xc2x0 C., and a vaporization temperature of 906xc2x0 C., whereas the steel forming the sheet typical as a melting temperature around 1530xc2x0 C. In order to weld the sheets, it is necessary to bring the steel of which they are constituted to melting, which means that the welding is carried out in the presence of zinc vapours. In the case of electric spot welding, the vapours do not give rise to any substantial drawbacks, since the steel of the sheets begins to melt at the contact area between the sheets (because this area is that having the highest electric resistivity) and due to that the welding electrodes apply also a pressure on the sheets which causes the expulsion from the contact area of the sheets of any quantities in excess of the melting bath and the zinc vapours. In the case of laser welding, instead, the melting bath starts to be created at the outer surface of the sheet where the laser beam is directed. As soon as the area of the melting bath, which typical assumes a keyhole-shaped form, extends down to the contact area between the sheets, the zinc vapours meanwhile created at this area escaped outwardly through the route of least resistance, constituted by the melting bath. Therefore, spray of melted metal occur which jeopardize the quality and strength of the welded joint, as well as the aesthetical appearance of the product thus obtained.
Various solutions have been proposed in the past in the endeavour to overcome the above-mentioned drawback. A first solution is to modify the shape of at least one of the sheets at the welding area so as to define a gap between the overlapped sheets which can be exploited by the zinc vapours to escape upwardly (see for example U.S. Pat. Nos. 4,682,002 and 4,916,284). A similar solution lies in providing spacing elements between the sheets, again in order to define a gap for evacuation of the zinc vapours (WO-A-99/08829, JP-A-5318155). All the above-indicated known solutions have the drawback however to require that an additional operation is carried out with respect to the conventional work cycle, since it is necessary to form the above-mentioned spacing deformed portions in the sheets or anyhow to apply additional elements thereto, which increases time and costs of production.
The object of the present invention is that of overcoming the above-described technical problem in a simple and efficient way.
In order to achieve this object, the invention provides a method for laser welding of two or more metal sheets which are overlapped to each other,
wherein at least one of the sheets has a protective layer on its surface in contact with the other sheet, said protective layer being of a material having a temperature of vaporization lower than the melting temperature of the material forming the sheet,
wherein the sheets to be welded are clamped to each other during the welding operation with the aid of clamping means which are movable between an inoperative condition and an operative condition in order to press the overlapped sheets against each other, and
wherein welding is carried out by directing a laser beam at the welding area of the clamped overlapped sheets,
said method being characterized in that:
at least one of the sheets is shaped or arranged so that when the clamping means are brought from their inoperative condition to their operative condition, in which they press the sheets against to each other, this sheet undergoes an elastic deformation from a rest condition in which a gap is defined between this sheet and the adjacent sheet, to a deformed condition in which this gap is nullified and the sheets are completely in contact with the each other at the welding area, and
in that before carrying out the welding operation, the clamping means are controlled according to the following sequence of stages:
in a first stage, the clamping means are brought from their inoperative condition to their operative condition, so as to insure the complete contact between the welding sheets through said elastic deformation,
in a second stage, the clamping means are moved back towards their inoperative condition, by a predetermined limited length, so as to create again a gap of substantially predetermined dimensions between the sheets to be welded, by exploiting the elastic return of said sheet which has been elastically deformed in the fist stage,
the clamping means are kept in the condition reached there by while welding is carried out, so that the vapours of protective material which are created during welding can escape through said gap.
Due to the above-mentioned features, the method according to the invention is able to successfully overcome the problem of the evacuation of zinc vapours which are created during laser welding of overlapped zinc-plated sheets. As in the case of the above-mentioned prior art, the vapours are let to escape through a gap defined between the sheets, so that the vapours are not encouraged to pass through the melting bath, which would give origin to the above-mentioned drawbacks. However, while in the case of the prior art the gap for evacuation of the vapours is obtained by a permanent deformation of at least one of the sheets before welding is carried out, or by the introduction of spacing elements between the sheets, in the case of the present invention the sheets are not subjected to any permanent deformation, nor spacing means are provided there between. As a matter of fact, in the case of the invention, the elastic return is simply exploited of at least one of the sheets from the condition of complete contact, in which this sheet is temporarily brought by the clamping means by exploiting the elastic deformability of the sheet, to a less deformed condition which is reached by partially opening again the clamping means until a predetermined position is obtained. In this manner, it is possible to ensure that a gap between the sheets of substantially predetermined dimensions is obtained, which is exploited by the zinc vapours to escape outwardly, without passing through the welding melting bath.
In the preferred embodiment of the invention, the above-mentioned clamping means are provided with sensor means adapted to detect when the condition of complete contact of the sheets is reached at the end of the above-mentioned first stage. These sensor means can be constituted by any type of direct or indirect sensor adapted to detect when this condition is reached, such as force or pressure sensor (which detects when a threshold value is passed) or a sensor of the speed of the movable part of the clamping means (which detects when this part is stopped) or a sensor of movement or a sensor of the electric current absorbed by the electric motor which controls the clamping device (in case this device is electrically actuated) or any other known type of sensor.
Also in the case of the above-mentioned preferred embodiment, the clamping means comprise at least a movable member which is movable between an inoperative position and an operative position by actuating means which are controlled in position.
Naturally, the invention is also directed to the welding system used in the method according to the invention,
said system comprising:
clamping means for clamping the sheets to be welded with each other while welding is carried out, said welding means being movable between an inoperative condition and an operative condition to press the overlapped sheets against each other, and
laser welding means adapted to direct a laser beam at the welding area of the clamped overlapped sheets,
said system being characterised in that:
at least one of the sheets is shaped or arranged so that when the clamping means are brought from their inoperative condition to their operative condition, in which said clamping means press the sheets against each other, this sheet undergoes an elastic deformation from a condition in which between this sheet and the adjacent sheet there is defined a gap, to a deformed condition, in which this gap is nullified and the sheets are completely in contact with each other at the welding area, and
in that said device further comprises actuated means for actuating said clamping means, which are controlled according to the sequence of stage which has been described above.
In the preferred embodiment of the system according to the invention, the clamping means are provided, as already indicated, with sensor means adapted to detect when the condition of complete contact of the sheets is reached, and said actuating means are controlled by control means which causes the clamping means to be stopped at the condition of complete contact of the sheet at the end of said first stage.
According to a further feature, the clamping means comprise at least a movable member which is movable between an inoperative position and an operative position, and said control means cause the movable member to be stopped at the end of said second step when they detect that a predetermined position is reached.
The actuating means may comprise an electric motor or a fluid actuator or a piezoelectric actuator or a shape memory alloy actuator or any other means.
According to a further aspect, the invention is also directed to the clamping device per se which is used in the method according to the invention. The clamping device according to the invention is characterised in that it comprises:
at least one clamping movable member movable between an inoperative condition and an operative condition to press the overlapped sheets against each other,
actuating means for actuating the clamping movable member,
sensor means adapted to detect a condition in which the clamping device holds the sheets in complete contact which each other,
control means which receive signals from said sensor means and controls actuating means so that:
in a first stage, the clamping member is brought from its inoperative condition to its operative condition and stopped when reaching is detected of the condition in which the clamping device keeps the sheets in complete contact which other,
in a second stage, the clamping member is moved back towards its inoperative condition, through a limited length, until reaching a predetermined position signalled by said sensor means.